1. Field of the Invention
This invention relates to organic light-emitting displays, and particularly to improved contrast organic light-emitting displays.
2. Description of the Related Art
Research proceeds on many fronts to develop high-resolution displays, for laptop computers, personal communicators and televisions, for example, that are thin, reliable, and low-cost. One technique being actively pursued makes use of the light-emitting properties of certain organic materials. These materials, such as aluminum tris 8-hydroxyquninoline (Alq.sub.3) , are sandwiched between anode and cathode electrodes, and emit light when a voltage is applied across the electrodes.
One organic-material-based display is discussed in P. May, "Light-Emitting Polymers: A Technology for Conformable Graphic Displays", SID 96 Digest, paper 14.4, pp. 192-195. The display is configured as a two-dimensional array of individually addressable pixels which form a passive-matrix display. The display's electrodes are formed as first and second layers of parallel strips, with a continuous sheet of organic material sandwiched between the two layers. The electrode strips of the upper layer are oriented perpendicular to the strips of the lower layer, and pixels are formed at the points where the perpendicular electrodes cross. The perpendicular electrode strips act as addressing lines: light is emitted from an individual pixel when a voltage is applied across its upper and lower electrodes so that a current passes through the organic material sandwiched between them.
This arrangement suffers from several problems, however, including poor contrast, optical and electrical crosstalk, and low brightness. Such a display suffers from poor contrast due to the reflective nature of the upper electrodes: the upper electrode strips serve as cathodes, and are made from magnesium-silver (Mg--Ag) a highly reflective material. Ambient light passing through the transparent substrate bounces off the reflective cathodes, which significantly limits the contrast between the pixel's "on state", i.e., when the pixel is emitting light, and its "off state".
Because the organic material is one continuous sheet, light spreads from one pixel region to another, resulting in optical crosstalk. The continuous nature of the organic material also leads to electrical crosstalk between pixels. Further crosstalk problems arise due to the fact that the emitted light travels from the organic material to the viewer through the lower electrode strips and through the substrate, requiring both the lower electrodes and the substrate to be a transparent material. The lower electrodes serve as anodes and are made from indium-tin-oxide (ITO), which has a refractive index of about 2.2. The substrate is typically glass, with a refractive index of about 1.5, and air has an index of 1. Due to these different indices, some light is trapped in both the ITO and glass layers, resulting in optical crosstalk and reduced efficiency.
Displays based on organic light-emitting materials are fabricated on glass substrates, with light emitted through the glass. However, because a glass substrate does not support the fabrication of crystalline silicon transistors (which require a silicon substrate), such displays are limited to the passive-matrix type, which have driving transistors placed at the end of each electrode strip/address line only. Active-matrix displays, on the other hand, which are generally preferred due to their superior brightness, place one or more driving transistors at each pixel; this provides for a greater current flow through and an increased brightness from each addressed pixel.
Some efforts have been directed towards display structures which emit light through a transparent upper electrode, as well as through a glass substrate. These displays, discussed for example in U.S. Pat. No. 4,885,211 to Tang et al. and in G. Gu, "Transparent organic light emitting devices," Appl. Phys. Lett., vol. 68, No. 19 (1996), pp. 2606-2608, are essentially "see-through" devices. However, as with the glass substrate devices discussed above, the transparent nature of the displays limits the amount of contrast which can be obtained between their pixels' on and off states.